The present invention relates to seismic bridges and, more particularly, to a seismic corridor bridge for bridging and providing pedestrian access across a seismic joint in a base isolated building.
In the construction of buildings, special regulations and codes have been enacted over the years to ensure that buildings can withstand a certain amount of stresses due to thermal changes and, depending on the geographic location, to also withstand vibrations and forces generated during an earthquake. In geographical areas where earthquakes generally do not occur, seismic activity is not a concern and, therefore, routine expansion joints are used to control the effects of linear expansion and contraction from thermal changes. In geographical areas where earthquakes are known to occur, however, seismic joints are used to control the effects of thermal changes and to accommodate the unpredictable movements associated with a seismic event.
There are basically two types of building construction, generally known in the industry as fixed base construction and base isolated construction. In fixed base construction, the lower end of building columns are bolted or otherwise fixed to footings supported directly by the ground. Fixed base construction may be designed to resist up-lift as well as to carry building load. In contrast, in base isolated construction, the lower end of building columns are bolted to isolators which are, in turn, bolted to footings supported by the ground. Base isolated columns are designed to support building load, but they are not designed to resist up-lift.
The isolators used in base isolated construction typically comprise alternately laminated layers of rubber compound and steel plates which are steam cured into a single unit and capped on the top and bottom with thicker steel plates. The lower ends of the building columns are bolted to the top plates of the isolators, while the bottom plates are bolted to the tops of concrete footings supported directly by the ground. The isolators are designed to displace horizontally in any direction by absorbing the energy imparted to them by an earthquake, but over a time duration that is much longer than the earthquake cycle duration. While the isolators cannot change the amount of force imparted to the building columns by the earthquake, they can increase the time period over which the earthquake force acts on the building column, thereby dissipating the seismic impact.
To fully appreciate the problem solved by the present invention, it is necessary to understand the nature and magnitude of the building movements that occur in fixed base construction and base isolated construction. As described below, the seismic joints in both types of construction must be able to accommodate routine building displacement due to thermal changes, as well as the building displacement associated with the seismic event. But first, a brief example of a building having an expansion joint will be provided as preliminary background.
As noted above, expansion joints are used in fixed base construction when there is no concern about the possibility of an earthquake. By way of example, in a 6 story steel building that is 300 feet wide and 600 feet long, the 600 foot length of the building will induce unwanted structural stresses and actual ruptures in finish materials, both interior and exterior, through the linear expansion and contraction caused by thermal changes. To control the effects of these thermal changes, an expansion joint will be used to separate the building structurally into two separate 300 square foot blocks that are adequately separated from each other, for example, by about 6 inches. Thus, if each of the 300 square foot blocks should expand at the roof line by 3 inches, to make the plan dimension of each block 300 feet 3 inches square, then an acceptable clearance of 3 inches would still remain in the expansion joint at the roof line of the building.
When a building having fixed base construction is located in an area known to have earthquakes, seismic joints are used instead of expansion joints. These seismic joints must be able to accommodate the same dimensional change due to thermal expansion of the type described above. In addition, the seismic joints must be able to accommodate seismic drift, also known as seismic sway. Seismic drift is the horizontal displacement or distortion of a building frame that occurs in resisting earthquake energy imparted to the building during a seismic event. In fixed base construction, the building displacement from seismic drift occurs at all floor levels and the roof above the first or ground floor. However, there is generally no displacement at the first floor, because the building columns are bolted to the footings supported directly by the ground. In a six story building, the seismic drift at the roof line may be as high as 9 inches relative to the ground floor. Therefore, in computing the clearance dimension that is necessary in a seismic joint under extreme seismic and thermal conditions, the width of the seismic joint under neutral conditions should include a 3 inch clearance under extreme thermal and seismic conditions, about 3 inches for thermal expansion, and about 18 inches for seismic drift (i.e., 9 inches for each part of the structure adjacent to the seismic joint, assuming the adjacent structures are moved toward each other). Thus, the total clearance dimension of the seismic joint in the neutral position is approximately 24 inches.
While the foregoing 24 inch calculation accounts for movement of adjacent structures perpendicular to the seismic joint, similar displacements parallel to the seismic joint are just as likely to occur. Moreover, if a seismic event causes the two parts of the structure adjacent to the seismic joint to move away from each other, without any thermal expansion, the 24 inch neutral seismic joint dimension would increase to approximately 42 inches (comprised of the 24 inch clearance dimension of the seismic joint in the neutral position as calculated above, plus an additional 18 inches of seismic drift of the structures away from each other).
Even further types of displacements must be accommodated in seismic joints in base isolated construction. The seismic joints in base isolated construction must be able to accommodate the displacements caused by thermal expansion and seismic drift described above, as well as displacement allowed by the isolators. The isolators allow building displacement to occur at all floor levels and the roof, including the first floor, which is elevated above the footing by the isolator but may be at the same elevation as the grade outside the building. However, in the six story building being used as our example, seismic drift may decrease from about 9 inches to about 3 inches since some of the earthquake energy is dissipated directly in distorting the isolators. In this regard, the isolators will be designed to support columns that face each other across the seismic joint, with the isolators for these columns being bolted to a common concrete footing. Assuming that the isolator will allow displacements horizontally in any direction from neutral in the range of about 24 inches, then the width of the seismic joint in the neutral position would be calculated by providing a 3 inch clearance under extreme thermal and seismic conditions, 3 inches for thermal expansion (as calculated above), an additional 6 inches to allow for seismic drift (assuming the adjacent structures move toward each other), and approximately 48 inches for isolator displacement under extreme seismic activity (24 inches for each part of the structure adjacent to the seismic joint, assuming the adjacent structures are moved toward each other). Thus, the total clearance dimension of the base isolated seismic joint in the neutral position is approximately 60 inches.
However, if the seismic event causes the two parts of the structure adjacent to the seismic joint to move away from each other, without any thermal expansion, then the 60 inch neutral seismic joint dimension would increase by another 54 inches under extreme seismic activity, now bringing the total seismic joint dimension to approximately 114 inches (comprised of 60 inches for the clearance dimension of the base isolated seismic joint in the neutral position, as calculated above, plus an additional 48 inches for displacement of the isolators away from each other, plus another 6 inches for seismic drift of the structures away from each other).
In the foregoing situations, where the seismic joint may expand to as much as 114 inches between adjacent building structures, very unusual and difficult problems are presented. For example, special care and consideration must be given in order to provide a bridge or walkway that provides constant internal cross-sectional dimensions and allows pedestrian access between the adjacent building structures on opposite sides of the seismic joint. Most certainly, the bridge should be designed to accommodate the range of relative movements, as described above, between the two building structures across the seismic joint. The bridge also should be designed to accommodate a range of such relative movements that is as wide as possible, without sacrificing the structural integrity of the bridge. Thus far, no satisfactory bridges have been developed to meet these design parameters.
Accordingly, there has existed a definite need for a seismic bridge that provides pedestrian access across a seismic joint separating two building structures, especially structures having base isolated construction, and that accommodates a relatively wide range of movements between the two building structures, such as during an earthquake, without compromising the structural integrity of the bridge. The seismic bridge of the present invention satisfies these and other needs and provides further related advantages.